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Generalized hydrodynamics in complete boxball system for $U_q(\widehat{sl}_n)$
by Atsuo Kuniba, Grégoire Misguich, Vincent Pasquier
This is not the current version.
Submission summary
As Contributors:  Grégoire Misguich 
Arxiv Link:  https://arxiv.org/abs/2011.08052v1 (pdf) 
Date submitted:  20201117 11:00 
Submitted by:  Misguich, Grégoire 
Submitted to:  SciPost Physics 
Academic field:  Physics 
Specialties: 

Approach:  Theoretical 
Abstract
We introduce the complete boxball system (cBBS), which is an integrable cellular automaton on 1D lattice associated with the quantum group $U_q(\widehat{sl}_n)$. Compared with the conventional $(n1)$color BBS, it enjoys a remarkable simplification that scattering of solitons is totally diagonal. We also introduce a randomized cBBS and study its nonequilibrium behavior by thermodynamic Bethe ansatz and generalized hydrodynamics. Excellent agreement is demonstrated between theoretical predictions and numerical simulation on the density plateaux generated from domain wall initial conditions including their diffusive broadening.
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Reports on this Submission
Anonymous Report 3 on 2021118 Invited Report
Strengths
 Presents a novel and very rich integrable model with an arbitrary rank underlying symmetry algebra, yet the model is simple and has only a finite spectrum of velocities of solitons
 The model has a very rich set of commuting dynamical automorphisms
 Inverse scattering formulated in a very explicit manner
 GHD equations of the model take remarkably simple form in terms of Y system of TBA
Weaknesses
 Some terminology is a bit unusual for statmech (see below)
 English needs a bit of polishing
Report
This is a remarkable paper!
It proposes a rich family of models, the socalled complete boxball systems, whose local phase space is defined on the product set of column semistandard Young tableauxs, and whose underlying symmetry algebra is given b U_q(sl_n).
The elementary solution excitaitons can be completely classified and shown to undergo purely diagonal scattering, a feature which largely simplifies hydrodynamic treatment of the model. The deterministic local time evolution map is given in terms of combinatorial R satisfying a YangBaxter equation with a discrete version of a spectral parameter. This is a beautiful discrete spacetimestate version of a higherrank integrable theory, and can serve either as a playground for developing nonequilirbium statistical mechanics of integrable systems, or as a cenceptually new kind of integrable systems.
Despite being very technical, the paper is not too hard to follow. It is clearly structured and quite well written. I enthusiastically recomment its publication in SciPost.
Requested changes
1 Is comutative diagram (2.48) completely correct, namely the two verstical mappingfs T^{(k)}_l are not identical. One is a conjugation of the other by the map \Phi. See also last intext formula on the page 16.
2 maybe notation on the RHS of (2.52) has to be explained?
3 I think it is an awkard nomenclature to call cBBS ystem’s configurations as “states”, and then treatment of distributions over configurations  which in the more standard language of statistical mechanics would correspond to “states”  as “randomized cBBS”. Maybe the authors can rethink that, but my suggestion would be to use “configurations”, for the former, and “states” for the later, while the system (cBBS) has no intrinsic randomness (it is deterministic in both cases) so it does not make sense to call it “randomized cBBS” in case where statistical ensembles of cBBS configurations are considered.
4 There were other integrable deterministic cellular automata in recent literature, where the interplay between ballistic and diffusive transport has been established, even beyond the hydrodynamic assumtptions. See e.g. Commun. Math. Phys. 371, 651 (2019), or PRL 119, 110603 (2017). I don't expect that such explicit results could be achieved for cBBS system, but perhaps the authors can comment on this and/or make make an appropriate link to this relevant literature.
Anonymous Report 2 on 2021112 Invited Report
Strengths
1Introduces a new $sl_n$ generalisation of the original boxball system that is both natural and considerably simpler than the existing generalisation. In particular the soliton Smatrix is diagonal, involving only phase shifts unlike the earlier generalisation.
2The simplicity of this model is exploited in developing both the GGETBA and GHD analysis of the model.
3The numerical simulation presents compelling evidence for the validity of the GHD description of the plateau arising in the n=3 domain wall problem.
Weaknesses
1The presentation of the numerics could benefit from polishing.
2It would be nice to see a comparison of the diffusive correction predicted in Section 5.2 with the observed broadening of the plateaus in the numerical simulation.
Report
The paper revisits the $sl_n$ generalisation of the TakahashiSatsuma BoxBall model. It develops a simpler and in many ways more natural generalisation than the existing one in the literature (described for example in [5]). As in [5] the timeevolution operator is formulated in terms of $sl_n$ combinatorial Rmatrices. However, in the current work the local quantumspace representations are changed and given by (1.2). The soliton description of the full quantum space is then elegantly described in terms of fixed $sl_n$ colour solitons of different lengths/amplitudes. These solitons have the particular nice property that their factorised Smatrix is diagonal  unlike the considerably more complex nondiagonal Smatrices of the previous $sl_n$ model. The Smatrix itself is very simple and is given by (2.40).
To digress:
this new $sl_n$ model should be useful in many contexts where a higherrank quantum integrable system with a simple and very explicit algebraic description is needed. In particular, it will be interesting to see how the parallel 'classical' description of this model works (that is the description of the latter Chapters of [5], including the associated ultradiscrete classical equation, tropical spectral curve, Jacobian, etc ). Hopefully the authors and their collaborators will develop this picture.
In the current paper the simplicity of the model is exploited in order to develop the GGETBA and GHD descriptions of the $sl_n$ model. I am not an expert in either of these approaches, but the presentation is clear and I can follow it to see how the plateau description given at the end of Section 5.2 arises.
In the final Sections of the paper, the authors carry out a numerical simulation of the $n=3$ model with 'domain wall' conditions and compare to the GHD description. The results themselves are qualitatively convincing  in that there is a clear coincidence of the predicted GHD plateaus and the smoothed plateaus of the numerics. I have some minor reservations about the presentation style of the numerics that I point out below. A more substantive point is that the numerical plateau are smoothed, and the obvious question is whether this smoothing can be described by the diffusive corrections to the ballistic description already developed in Section 5.2.
Overall I think this is a very nice paper, and that the $sl_n$ generalisation of the BallBox model is a useful contribution to the field in itself. Of course the paper goes way beyond this and develops the GHD description of the model and compares with numerics. The consistency with the numerics provides more convincing evidence of the validity of the GHD description of such quantum integrable systems.
Requested changes
1In Section 5.3, I can't see the difference between the different $t$ plots (even on my 'retina' display). While I understand that this is the point  it seems like bad practice to graphically represent two sets of data in a way where you can't properly see either because of overlap with the other. I leave it to the authors to consider if they can solve this issue.
2 Minor point, but the figures are referred to variously as fig 1, Fig. 1, Figure 1. Please be consistent.
3 There seems to be a problem with the ordering of figures: the discussion of Figure 10 comes directly after the discussion of Figure 6. Please fix.
4 In Section 6 it is stated that '[...] the simulations match perfectly the GHD expectations'. Either make this quantitive (with a percentage error) or don't state it.
5Section 6 gives a nice summary of what is a fairly long and technical paper. It would help with the readability of the paper if the authors could crossreference (i.e. include equation numbers for) the earlier results in the paper they are referring to in this concluding section.
6I think there deserves to be some discussion of the role of the diffusive corrections presented in Section 5.2 in the comparison with the numerics.
Anonymous Report 1 on 20201221 Invited Report
Strengths
1 a new integrable cellular automaton with nice scattering property is introduced
2 comprehensive analytical and numerical study of the model
3 very accurate and comprehensive numerical verification of generalised hyrdodynamics
Weaknesses
1 slightly technical
Report
In this paper, the authors study a new cellular automaton called the complete boxball system (cBBS). This is a model (in fact, this is a family of models) is based on the $U_q(\hat{sl}_n)$ algebra, and is a particular case of a large family of models that has been considered the literature. The cBBS itself has not been studied before. It is shown by the authors to have very specific and appealing dynamical properties, the main one being the fact solitons exist and scatter in a diagonal fashion. The existence of solitons in other BBSs has been seen, but this seems to be the first time a BBS is found to have diagonal scattering.
This offers a very good playground where non equilibrium dynamics can be studied. The TBA and GHD for this model is constructed and compared with numerical simulations. The agreement with GHD for the partitioning protocol is strikingly good, both for the Euler and diffusive GHD predictions. This is not only an interesting new integrable model to study, but also gives one of the most accurate numerical verifications of GHD that has been done until now.
The paper is well written. The model is very involved and its construction relatively technical, but the authors have done a very good work in explaining it and putting it in the context of the boxball systems. The GHD is well constructed, and the numerical analysis is excellent.
I do not have any particular request concerning the paper, and I believe it can be published as it is.
But on the science side, the one thing I am curious about is what is the correction to the diffusive behaviour seen in the numerics. It is mentioned that the diffusive GHD prediction is not quite reached by the numerics, but that the error decreases. How does it decrease? With a power law? Given how accurately GHD can be recovered, this would be an interesting thing to observe in order to guide future work on corrections beyond diffusive. Maybe the authors can make a quick remark about this?
Small thing:
Page 3: the dot before “non diagonal”?
monotonous > monotonic
I have only some general comments on the paper.
The boxball system is a rather degenerate classical integrable manybody
system, which has been mostly studied in Japan, but also more recently by Pablo
Ferrari, who is a well known probabilist. The submitted paper comes with a new twist,
namely internal degrees of freedom, which for computational reasons are taken to form
a quantum group. The authors have a previous recent paper on the standard boxball with
no internal degrees of freedom. The interesting and novel approach is to use the boxball
system as a testing ground for generalised hydrodynamics. They derive the equations and
arrive at predictions for particular initial data of domain wall type. The results are
compared with extensive numerical simulations
of the box ball system.
The study is timely and of considerable interest, by teaching us more about GHD in
the context of a concrete model. I strongly support publication.
The paper contains rather lengthy computations, which are unavoidable. But I am
not in a position to check the details. I suggest to ask T. Prosen. He has Ph.D. and
more advanced collaborators, which are well experienced in classical cellular
automata. They might be interested to understand some details of the
submitted paper. An additional option is Makiko Sasada, who is more on
the mathematical side (in this case presumably welcome). She is an expert
on boxball.